A stationary, aft foil oriented horizontally below the hull of a vessel, such as described in WO2004020276 A1, WO2007148966 A2 and/or WO2016010423 A1 by the present applicant, does not make optimal use of the energy that is present in the flow in developing thrust. At the location where the aft foil is fitted to ships (near the stern of the vessel) the flow is inclined aft and upwards. When the aft foil is positioned in this flow a lift force is developed at right angles thereto such that the horizontal component thereof constitutes a thrust force. The aft foil as disclosed in the above patent publications is presently optimized for the flow that is present when the vessel operates in calm water.
However, the vessel rarely operates in still water. The waves that are nearly always present along the routes that ships adopt cause the vessel to pitch, heave and roll. The effect thereof on the angle of inflow at the aft foil is significant, causing a large variation in the angle of incidence. When the depth of the aft foil below the water surface decreases (when a wave trough is present above the aft foil) the flow experiences an additional downward velocity component which reduces the angle of incidence, and when the depth of the aft foil below the surface increases (in the presence of a wave peak) the flow experiences an additional upwards velocity component which increases the angle of incidence.
For example, when a ship with a length of 100 m displays a pitching motion with a period of 5 seconds in seas which cause an average undulation of the water surface of 1 m at the location of the aft foil, when the horizontal component of the inflow is 7 m/s (for a ship speed of about 15 knots), the variation in the angle of incidence is some ±3.3 degrees and when the undulation is 2 m, about ±6.6 degrees. In actual fact this variation is about 60% higher because the vertical velocity component is not constant during one wave period but similar to a sine or cosine function. Furthermore, a ship rarely displays a pure pitching motion. Usually it also heaves and rolls and these motions also influence the direction of the inflow near the stern of a vessel. Thus, to constantly obtain the best lift-to-drag ratio from the aft foil—or the maximum thrust (whichever might be the case), the angle of attack of the foil needs to be constantly adapted to the direction of the inflow below the hull. An object of the invention is thus to provide an aforementioned vessel with an aft foil for generating a lift force—with a forwardly directed thrust component—wherein the angle of attack of the foil can be (constantly) adapted to the direction of the inflow below the hull.